Ductless, or filtration, fume hoods are a specific type of fume hood that use a filtration system to remove contaminants from an air stream. Ductless hoods operate by simply forcing contaminated air from the hood enclosure through a filter to remove particulates and chemical vapors before returning the air to the laboratory environment.
Ductless hoods have several convenient advantages over laboratory installed, ducted hoods. They are mobile and portable and have minimal installation costs as they do not have to be connected to a duct system. They are environmentally friendly, as no toxic gases are released into the atmosphere. Ductless hoods also have very low operating costs, as no conditioned air is removed from the laboratory and vented outside. Instead, the exhaust air is recirculated through the filter system into the laboratory environment.
Because of the advantages listed above, ductless hoods are popular with academic laboratories and hospitals. As ductless hoods are able to be operated anywhere in the laboratory and often are made transparent on all sides and as such, they are ideal for teaching demonstrations, allowing students to surround the hood. Ductless hoods have also grown in popularity in industrial laboratories, where they can be used for specific projects with low costs.
The main drawback of ductless hoods is the potential release of toxic gases into the laboratory because of filter saturation and breakthrough. While most of the advantages of using ductless hoods are derived from the re-circulating of air from the hood back into the laboratory, this re-circulation means that the air exiting the hood must be filtered and monitored at all times. Most ductless hoods use a filter comprising activated carbon as their filtration system. Although activated carbon is highly adsorbent, the activated carbon eventually become saturated. When the activated carbon becomes saturated through the thickness of the filter, chemical vapors are no longer adsorbed and begin to break through into the exhaust.
The primary inconvenience of operating a ductless hood is the need to monitor the hood filter to ensure the safety of those working in the laboratory. If the exhaust concentration of a specific compound exceeds the allowed limits set by the United States Occupational Health and Safety Administration (OSHA) or other local limits, then the filter must be changed. Prior designs have conventionally employed a timer that sounds an alarm every given number of days, typically between six and twelve months of operation time, to notify the user that it is time to check the condition of the filter. However, studies have shown that these arbitrary alarms rarely coincide with the actual timing of filter saturation. This is not surprising, as this arbitrary method of filter monitoring does not take into account the actual use of the hood while it is running. Because of the unreliability of the conventional alarms in the prior art, they are often ignored, leaving the user to test the condition of the filter whenever the user feels it might be necessary. In most cases, the user will wait until a detectable odor develops in the laboratory, which is often the point at which the concentration of the compound in the air has already exceeded the OSHA limits. In other cases where the compound being used is odorless, the user is forced to be very vigilant in checking filter efficiency, and often spends a great deal of time performing tests that are not necessary. Better methods of filter monitoring are needed to maintain the safety of the hood operator and others in the laboratory without imposing inconvenient requirements that the filter be checked more often than necessary.
Other ductless hood designs have attempted to solve this problem by installing a gas sensor downstream of the filter to detect the concentration of compounds in the filter exhaust. This effort has largely proven futile as it is not possible to monitor the hundreds of different compounds used in a laboratory with just one sensor. The sensors used in ductless hoods are typically broad range detectors without any specificity for particular compounds. Although control system read-out can be obtained for the exhaust gas concentration, it is difficult to correlate this read-out to an actual concentration of an actual gas, and report to the user if that concentration actually exceeds the OSHA exposure limits. Much of this phenomenon comes from the fact that many sensors have widely varied sensitivity to different gases. This varied sensitivity makes it very difficult to choose a level of detection for the sensor at which the alarm should be triggered, especially in a situation when multiple types of chemicals are to be used in the hood. Another major problem is that none of these alarm methods use an absolute zero reference so resulting alarm points are arbitrary. An improved ductless hood filter monitoring system would greatly improve on the safety and ease of use of ductless hoods.
Furthermore, using existing technology, it is still necessary to perform air sampling tests to confirm that the filter is actually compromised. These tests usually involve use of a gas detection tube containing a color change reagent specific to the gas to be detected. Whenever a filter alarm sounds, the user must stop work and take the time to sample the exhaust air using a hand pump before deciding if a filter change is actually necessary. More convenient methods for determining filter life are necessary to simplify compliance with safety regulations.